Analog constant current sources or pulsed analog constant current sources using linear dissipative pass elements have been used as diode drivers to power light emitting diodes, often laser diodes. FIG. 1 illustrates an array of LEDs (or diode array) that are connected to a power source. A linear control (pass) element is disposed in the return path from the LED array to the power source. Current flowing through the LEDs flows through a current sense resistor which supplies a voltage indicative of current to an input of an error amplifier, the other input of which receives a reference demand voltage indicative of the desires current. The output of the amplifier controls the linear control element to maintain a constant current through the LEDs. This is a simple, straightforward analog control loop. Such analog current sources are inefficient due to power (e.g., heat) dissipation in the linear pass element controlling the current.
U.S. Pat. No. 5,287,372 (“Ortiz”) discloses a quasi-resonant diode drive current source that provides high power pulsed current that drives light emitting diodes, and the like. The pulsed output current of the quasi-resonant diode drive current source is sensed, and is regulated by a control loop to a level required by the light emitting diodes. The implementation is heavy and not suited to smaller surface-mount technology because the power components are large to filter the single-phase switching waveform from the switch transistor (24). Normally, in either case (i.e., either of FIG. 1 or “Ortiz”) the switch (or pass) transistor would require a heat sink.
For pulsed laser or LED sources, the energy is typically stored in a capacitor to minimize a sudden lossy power demand from the prime power source. With a linear current regulator, the regulator pass element must remain in the linear region during the discharge of the energy storage capacitor to regulate the pulse of current. To minimize the voltage initially across this pass element, and hence dissipation, the capacitor must have a small value of voltage droop during this current draw, requiring a large amount of stored energy and a large capacitance value. The quasi-resonant controller shown by Ortiz is required to pass this peak current and power demand through to its power source (22), causing a large transient current draw, and is limited in rise and fall times which can be achieved due to the frequency and power limitation of a single stage and the time required to charge up the output filter capacitor (C1).